Generally, a linear motor is provided with a primary member and a secondary member. The primary member has a plurality of magnetic pole teeth. The secondary member is supported by the primary member through a gap so that it can move relatively with respect to the primary member. An electromagnetic coil is wound commonly on a plurality of the same polarity teeth so that the linear motor is reduced in size. Such a linear motor is disclosed in the official gazette of JP-A No.174418/1998 (hereinafter, to be referred to as the patent document 1). The official gazette of JP-A No. 28875/2001 (hereinafter, to be referred to as the patent document 2) also discloses another linear motor that is further reduced in size by winding an electromagnetic coil on all of the teeth of both magnetic polarities.
Furthermore, the official gazette of JP-A No. 85045/61 (hereinafter, to be referred to as the patent document 3) discloses still another linear motor in which a permanent magnet is disposed between magnetic poles to reduce the leak magnetic field.    [Patent document 1] Official gazette of JP-A No. 177418/1998    [Patent document 2] Official gazette of JP-A No. 28875/2001    [Patent document 3] Official gazette of JP-A No. 85045/1986
According to the linear motors disclosed in the patent documents 1 and 2, when a driving current flows in the electromagnetic coil wound on the core, the magnetic flux density in the core increases to cause magnetic saturation. In such a state, the core cannot lead the magnetic flux effectively any longer, thereby the thrust force does not increase how much the driving current increases. This is because the electromagnetic coil is wound commonly on the plurality of magnetic pole teeth, thereby the coil comes to go away from the end of the magnetic pole teeth.
According to the linear motor disclosed in the patent document 3, in which a permanent magnet is disposed between magnetic poles to reduce the leak magnetic field, the use of such permanent magnets is limited only when no change with time occurs in the direction of the leak magnetic field. Generally, the direction of the leak magnetic field changes with time. This is why the use of permanent magnets is not proper for linear motors.